1. Field of the Invention
The present invention relates to improvements in the temperature sensitivity of SAW based sensors for measuring both torque and temperature.
2. The Prior Art
Applicant's own earlier British Patent Application no. 2426336 discloses a contactless SAW based torque and temperature sensor comprising a first and a second SAW resonator provided on a substrate made of Y+34° cut of quartz. The first SAW has its principle axis inclined at +45° to the X-axis of the substrate, which, in use, is either aligned with the longitudinal axis of the device whose torque is to be measured or is perpendicular thereto, and the second SAW has its principle axis inclined at −45° to the X-axis of the substrate. A third SAW is also present having its principle axis inclined at an angle to the principle axis of both the first and second resonators.
In the first and the third embodiments shown in FIGS. 1 and 6 of GB 2426336, the SAW resonators for torque measurement M1SAW and M2SAW are positioned on the same substrate as the resonator TSAW (or resonators T1SAW and T2SAW) used for temperature measurement. The substrate is made of Y+34° cut quartz, and the angle between the SAW propagation direction for TSAW (T1SAW and T2SAW) and the X axis of the substrate α is close to 30° in order to maximize sensitivity of the frequency difference Ft to temperature (Ft is specified in T0060, for the first embodiment it equals the difference between the resonant frequencies of M2SAW and TSAW).
The pattern of the SAW resonators on the quartz substrate is usually made of a thin aluminum film. GB 2426336 is silent on the thickness h of the Al film and the variation of the frequency difference Ft with temperature presented in FIG. 3 of GB 2426336 is theoretically calculated for h=0. It corresponded to the average temperature sensitivity of approximately 9 kHz/° C.
In reality the Al film thickness has a finite value, with fabricated SAW Sensors, which have been publicly available, having an approximate Al film thickness of h=260 nm and resonant frequencies of M1SAW, M2SAW and TSAW equal to 437 MHz, 435 MHz and 433 MHz respectively.
Since the Al electrodes and reflecting strips modified properties of the quartz substrate surface, the actual temperature sensitivity of Ft in the prior art system turned out to be only 2.4 kHz/° C. that is considerably smaller than the theoretically predicted value of 9 kHz/° C.